3 Theories of the Origin of Life In the 1950s, Stanley Miller and Harold Urey set up an experimental “primitive” atmosphere and used a spark to simulate

3 Theories of the Origin of Life

• In the 1950s, Stanley Miller and Harold Urey set up an experimental “primitive” atmosphere and used a spark to simulate lightning.

• Within days, the system contained numerous complex molecules.

Figure 3.1 Synthesis of Prebiotic Molecules in an Experimental Atmosphere

3 Macromolecules: Giant Polymers

• There are four major types of biological macromolecules:

Proteins

Carbohydrates

Lipids

Nucleic acids


• Macromolecules are giant polymers.

• Polymers are formed by covalent linkages of smaller units called monomers.

• Molecules with molecular weights greater than 1,000 daltons (atomic mass units) are usually classified as macromolecules.


• The functions of macromolecules are related to their shape and the chemical properties of their monomers.

• Some of the roles of macromolecules include: Energy storage Structural support Transport Protection and defense Regulation of metabolic activities Means for movement, growth, and development Heredity

3 Condensation and Hydrolysis Reactions

• Macromolecules are made from smaller monomers by means of a condensation or dehydration reaction.

• Energy must be added to make or break a polymer.

• The reverse reaction, in which polymers are broken back into monomers, is a called a hydrolysis reaction.

Figure 3.3 Condensation and Hydrolysis of Polymers (Part 1)

Figure 3.3 Condensation and Hydrolysis of Polymers (Part 2)

3 Proteins: Polymers of Amino Acids

• Proteins are polymers of amino acids. They are molecules with diverse structures and functions.

• Proteins range in size from a few amino acids to thousands of them.

• Folding is crucial to the function of a protein and is influenced largely by the sequence of amino acids.


• An amino acid has four groups attached to a central carbon atom:

A hydrogen atom

An amino group (NH3+)

The acid is a carboxyl group (COO–).

Differences in amino acids come from the side chains, or the R groups.


• Amino acids can be classified based on the characteristics of their R groups.

Five have charged hydrophilic side chains.

Five have polar but uncharged side chains.

Seven have nonpolar hydrophobic side chains.

Cysteine has a terminal disulfide (—S—S—).

Glycine has a hydrogen atom as the R group.

Proline has a modified amino group that forms a covalent bond with the R group, forming a ring.

Table 3.2 The Twenty Amino Acids Found in Proteins (Part 1)




• Proteins are synthesized by condensation reactions between the amino group of one amino acid and the carboxyl group of another. This forms a peptide linkage.

• Proteins are also called polypeptides. A dipeptide is two amino acids long; a tripeptide, three. A polypeptide is multiple amino acids long.

Figure 3.5 Formation of Peptide Linkages


• There are four levels of protein structure: primary, secondary, tertiary, and quaternary.

• The precise sequence of amino acids is called its primary structure.

• Enormous numbers of different proteins are possible.


• A protein’s secondary structure consists of regular, repeated patterns in different regions in the polypeptide chain.

• The two common secondary structures are the helix and the pleated sheet.

Figure 3.6 The Four Levels of Protein Structure (Part 1)




• Tertiary structure is the three-dimensional shape of the completed polypeptide.

• Other factors can include the location of disulfide bridges, which form between cysteine residues.

Figure 3.4 A Disulfide Bridge


• Other factors determining tertiary structure:

Hydrophobic side-chain aggregation and van der Waals forces.

The ionic interactions between the positive and negative charges deep in the protein, away from water


• It is now possible to determine the complete description of a protein’s tertiary structure.

• The location of every atom in the molecule is specified in three-dimensional space.


• Quaternary structure results from the ways in which multiple polypeptide subunits bind together and interact.

• Hemoglobin is an example of such a protein; it has four subunits.

Figure 3.8 Quaternary Structure of a Protein


• Shape is crucial to the functioning of some proteins:

Enzymes need certain surface shapes in order to bind substrates correctly.

Carrier proteins in the cell surface membrane allow substances to enter the cell.

Chemical signals such as hormones bind to proteins on the cell surface membrane.


• Changes in temperature, pH, salt concentrations, and oxidation or reduction conditions can change the shape of proteins.

• This loss of a protein’s normal three-dimensional structure is called denaturation.

3 Carbohydrates: Sugars and Sugar Polymers

• Carbohydrates are carbon molecules with hydrogen and hydroxyl groups.

• They act as energy storage and transport molecules.

• They also serve as structural components.


• There are four major categories of carbohydrates:

Monosaccharides

Disaccharides, which consist of two monosaccharides

Oligosaccharides, which consist of between 3 and 20 monosaccharides

Polysaccharides, which are composed of hundreds to hundreds of thousands of monosaccharides


• All living cells contain the monosaccharide glucose (C6H12O6).

• Glucose exists as a straight chain and a ring, with the ring form predominant.


• Different monosaccharides have different numbers or different arrangements of carbons.

• Most monosaccharides are optical isomers.

• Hexoses (six-carbon sugars) include the structural isomers glucose, fructose, mannose, and galactose.

• Pentoses are five-carbon sugars.

Figure 3.14 Monosaccharides Are Simple Sugars (Part 1)

Figure 3.14 Monosaccharides Are Simple Sugars (Part 2)


• Monosaccharides are bonded together covalently by condensation reactions. The bonds are called glycosidic linkages.

• Disaccharides have just one such linkage: sucrose, lactose, maltose, cellobiose.

Figure 3.15 Disaccharides Are Formed by Glycosidic Linkages


• Oligosaccharides contain more than two monosaccharides.

• Many proteins found on the outer surface of cells have oligosaccharides attached to the R group of certain amino acids, or to lipids.

• The human ABO blood types owe their specificity to oligosaccharide chains.


• Polysaccharides are giant polymers of monosaccharides connected by glycosidic linkages.

• Cellulose is a giant polymer of glucose joined by -1,4 linkages.

• Starch is a polysaccharide of glucose with -1,4 linkages.


• Carbohydrates are modified by the addition of functional groups:

Glucose can acquire a carboxyl group (—COOH), forming glucuronic acid.

Phosphate added to one or more hydroxyl (—OH) sites creates a sugar phosphate such as fructose 1,6-bisphosphate.

Amino groups can be substituted for —OH groups, making amino sugars such as glucosamine and galactosamine.

3 Lipids: Water-Insoluble Molecules

• Lipids are insoluble in water.

• This insolubility results from the many nonpolar covalent bonds of hydrogen and carbon in lipids.


• Roles for lipids in organisms include:

Energy storage (fats and oils)

Cell membranes (phospholipids)

Capture of light energy (carotinoids)

Hormones and vitamins (steroids and modified fatty acids)

Thermal insulation

Electrical insulation of nerves

Water repellency (waxes and oils)


• Fats and oils store energy.

• Fats and oils are triglycerides, composed of three fatty acid molecules and one glycerol molecule.

Figure 3.18 Synthesis of a Triglyceride


• Saturated fatty acids have only single carbon-to-carbon bonds and are said to be saturated with hydrogens.

• Saturated fatty acids are rigid and straight, and solid at room temperature. Animal fats are saturated.


• Unsaturated fatty acids have at least one double-bonded carbon in one of the chains —the chain is not completely saturated with hydrogen atoms.

• The double bonds cause kinks that prevent easy packing. Unsaturated fatty acids are liquid at room temperature. Plants commonly have unsaturated fatty acids.


• Phospholipids have two hydrophobic fatty acid tails and one hydrophilic phosphate group attached to the glycerol.

• As a result, phospholipids orient themselves so that the phosphate group faces water and the tail faces away.

• In aqueous environments, these lipids form bilayers, with heads facing outward, tails facing inward. Cell membranes are structured this way.

Figure 3.21 Phospholipids Form a Bilayer


• Steroids are signaling molecules.

• Steroids are organic compounds with a series of fused rings.

• The steroid cholesterol is a common part of animal cell membranes.

• Cholesterol is also is an initial substrate for synthesis of the hormones testosterone and estrogen.

Figure 3.23 All Steroids Have the Same Ring Structure

3 Nucleic Acids: Informational MacromoleculesThat Can Be Catalytic

• Nucleic acids are polymers that are specialized for storage and transmission of information.

• Two types of nucleic acid are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• DNA encodes hereditary information and transfers information to RNA molecules.

• The information in RNA is decoded to specify the sequence of amino acids in proteins.


• Nucleic acids are polymers of nucleotides.

• A nucleotide consists of a pentose sugar, a phosphate group, and a nitrogen-containing base.

• In DNA, the pentose sugar is deoxyribose; in RNA it is ribose.

Figure 3.24 Nucleotides Have Three Components


• DNA typically is double-stranded.

• The two separate polymer chains are held together by hydrogen bonding between their nitrogenous bases.

• The base pairing is complementary: At each position where a purine is found on one strand, a pyrimidine is found on the other.

• Purines have a double-ring structure. Pyrimidines have one ring.

Figure 3.25 Distinguishing Characteristics of DNA and RNA


• The linkages that hold the nucleotides in RNA and DNA are called phosphodiester linkages.

• In DNA, the two strands are antiparallel.

• The DNA strands form a double helix, a molecule with a right-hand twist.


• Most RNA molecules consist of only a single polynucleotide chain.

• Instead of the base thymine, RNA uses the base uracil.

• Hydrogen bonding between ribonucleotides in RNA can result in complex three-dimensional shapes.

Figure 3.26 Hydrogen Bonding in RNA


• DNA is an information molecule. The information is stored in the order of the four different bases.

• This order is transferred to RNA molecules, which are used to direct the order of the amino acids in proteins.


• Closely related living species have DNA base sequences that are more similar than distantly related species.

• The comparative study of base sequences has confirmed many of the traditional classifications of organisms.

• DNA comparisons confirm that our closest living relatives are chimpanzees: We share more than 98 percent of our DNA base sequences.


• Nucleotides have other important roles:

The ribonucleotide ATP acts as an energy transducer in many biochemical reactions.

The ribonucleotide GTP powers protein synthesis.

cAMP (cyclic AMP) is a special ribonucleotide that is essential for hormone action and the transfer of information by the nervous system.

Figure 3.28 Disproving the Spontaneous Generation of Life (Part 1)

Figure 3.28 Disproving the Spontaneous Generation of Life (Part 2)

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3 Theories of the Origin of Life In the 1950s, Stanley Miller and Harold Urey set up an experimental “primitive” atmosphere and used a spark to simulate